Big Sway Bar, Soft Springs Setups – BBSS

The Trend Of Using A Big Sway Bar And Soft Front Springs Has Swept Over The Short-Track Industry

There is a setup trend that has taken hold in circle track racing over the past several years in which teams have been using much softer front spring rates and very large-diameter sway bars. Some teams have had a measure of success while others have become frustrated trying to make it all work for them. Before softening up your setup and sticking in a large sway bar, let’s look at how the sway bar works, what influences the bar has on the setup, and where we can possibly utilize this combination.

When racers across the country were asked if lap records have fallen since teams have switched to the big bar/soft springs (BBSS) setups, the general consensus was that lap times are about the same as before for the fast cars. If that is true, then why bother?

In other conversations, racers tell me an inexperienced driver can generally do better with the BBSS setups than trying to figure out the more conventional setups. That being said, the majority of all setups suffer from an imbalance anyhow, and maybe the BBSS setups bring the car closer to a balanced state than ever before for some teams.

Jay Fogleman is a top Hooters ProCup driver who has a great deal of short-track experience in the Late Model divisions. Fogleman thinks that with the BBSS setups, along with the stiffer right-rear spring, the car already feels set on initial entry without having the driver wait for the car to take a set. This lets the driver get into the throttle sooner, whether that is an improvement or not. The bottom line is that, in some cases, it makes the driver feel more confident.

Since opinions vary, it gives us something to think about and play with. After all, racers were born to experiment, and for most circle track racers, the search for the ultimate setup is nearly as much fun as the results. Let’s take a good look at what sway bars do, some general characteristics of sway bars, and how we can utilize the BBSS setups.

Definition A sway bar is, in effect, a type of torsion bar designed to act as a mechanical resistance to body sway or roll. As the car rolls, the bar must twist, and the stiffness of the bar, or resistance to twisting, determines its effect. Most asphalt cars have sway bars in the front suspension and sometimes in the rear suspension. Using a sway bar means softer springs can be used without the negative effects of excess body roll.

One type of sway bar seen on a circle track car is the one-piece tubular or solid type that resembles the stock sway bars that come on factory passenger cars. On the Nextel Cup cars, and increasingly installed on Late Model Stock cars, are the straight bars with arms attached over splined ends of the sway bar.

Sway bars are rated by the pounds of resistance per inch of arm movement of each arm. A bar rated at 300 pounds per inch means that if the bar is secured and the end of the arm is moved 1 inch, we would measure 300 pounds of force or load if the end of the arm were resting on a spring rater.

A coil spring is merely a sway bar that is coiled. When we compress a coil spring, the resistance is created by the wire twisting exactly as it does in a sway bar. The amount of resistance is related to the diameter of the wire, the length of that portion of the wire that is allowed to twist, the length of the connecting arm, and the stiffness of the metal, usually referred to as the shear modulus.

The shear modulus (SM) is different for various types of metals. It is represented by a number multiplied by 10 to the sixth power to determine the pounds per square inch (psi) rating of the metal. The number for cold-rolled steel is 11.5. Heat-treated carbon steel is rated from 11 to 11.9, and stainless steel is rated at around 10.6. It is almost impossible to know for sure what the SM is for a particular bar, but, for our calculations, we can use an average number of 11.5, which would be an average of the various metals used.

The rate of the sway bar is affected by the construction of the arm and how much that arm will bend when under a load. Aluminum arms will bend quite a bit, especially with larger-diameter bars. This affects the net rate of the bar as felt by the car. Although steel arms are heavier, they are much better to use with a large sway bar than aluminum when bending is a problem.

The resistance to twisting in a bar is the greatest at the outer portion if we view the bar in a cross section. The middle of the bar offers very little resistance, just as a small bar is easy to twist. If we take a 111/44-inch bar and gun-drill out the middle, we will have a much lighter bar with almost as much resistance to twisting as the original solid bar. If we drill out a 31/44-inch hole, the thickness of the wall of the sway bar is now 11/44 inch. That outer 11/44 inch of the bar has much more resistance than the inner 31/44 inch.

Using an example of a bar with 32 inches of effective length with 12-inch arms, a standard metal 111/44-inch sway bar has a rate of 597 pounds per inch of arm movement. In contrast, a 31/44-inch sway bar has a rate, using the same length and arms, of only 77 pounds. The rate of sway bars increases in an algebraic fashion. While a 1-inch bar is only rated at 244 pounds per inch, a 2-inch bar is rated at an incredible 3,911 pounds per inch.

The length of the sway bar arms does affect the rate of the bar. A shorter arm results in a stiffer overall rate at the end of the arm than a longer arm. For example, a 10-inch arm on a 1-inch sway bar yields a rate of 293 pounds per inch. If we increase the length of the arm to 12 inches, we see a reduced rate of 244 pounds per inch.

How to Mount Sway Bars The sway bar should be free to twist with no binding. The attachment where the sway bar is connected to the chassis should be free of any restrictions so that bar can twist freely as the chassis rolls. The NASCAR-type sway bar is mounted in a tube with bearings at each end so the bar is free to rotate without restriction.

The arms should be clamped tight to the bar if using a straight-style sway bar. Where the bar is attached to the lower control arm, the connection should be positioned at right angles to the sway bar arm as well as the lower control arm. Any angle in the link can greatly increase the “felt” rate of the bar.

Sway Bar Preload One benefit of using a sway bar is that it helps us get more bite off the corners. Sway bar preload, actually pre-twisting the bar, increases this bite effect. It also causes an increase in the crossweight percentage. It is best to put in preload and then set the correct crossweight in the car. That is opposite the way teams have usually done their sway bar preload, but we need to know exactly how much crossweight is in the car when we race it. If we set the crossweight and then preload the bar, we don’t know where our crossweight is.

One of the drawbacks of using a large-diameter sway bar is that we cannot preload the bar. If we try, the whole chassis will rotate, or roll left, and the ride heights will go crazy. I have watched a team fight forever to get the proper ride heights while preloading the large bar they had installed. The bottom line is that we need no preload on a large-diameter bar. The larger size provides the effect without using preload.

How Sway Bars Affect Our Setups In past articles on chassis setup, we have stressed that our two suspension systems must be balanced and working together. We design our suspensions for spring rate and moment center location so that both ends of the car are working in harmony. The end result of a balanced chassis is that each end of the car tends to roll to the very same angle in the turns. Since the sway bar adds resistance to chassis roll, it has an effect on the balance of the car.

If we add a sway bar to our car, it has the same effect on chassis roll as adding wheel rate to the car. The car senses the spring and sway bar rates at the wheel, and those rates are reduced by the motion ratio or installed ratio. So we need to translate the sway bar rate to a rate at the wheel to understand the true effect. This is an important point, because if we mount the sway bar arm closer to the ball joint, we end up with a stiffer sway bar rate at the wheel. A sway bar arm mounted inboard toward the inner chassis mounts of the lower control arm has a much softer rate.

Going Big on Bars and Soft on Springs When we install a large-diameter bar, we are effectively adding spring rate to our cars in a big way. Let’s say we have a touring Late Model car that has 300-pound springs in the frontend. If we install a 111/42-inch sway bar, the effect on reducing body roll is just the same as if we added 1,237 pounds of spring rate to the car. We could install 1,550-pound springs in the front of the car and have the same diminishing effect on body roll.

The effect on body dive is not increased when using a large-diameter sway bar. So we may be able to increase the aero downforce on our cars by installing softer springs with the BBSS setups. The car will compress in the turns instead of rolling, and the overall attitude of the car will be lower. With a lower-sprung mass, the center of gravity will be lower and less weight will transfer from the left-side tires onto the right-side tires. More retained left-side weight means more overall traction. A lower body profile means less drag, more overall downforce, and more load on the tires.

This all seems like a win-win situation. It can be, except for one fact: We still need a balanced setup in order for our car to be consistently fast. When we reduce the front roll angle to a fraction of the old roll angle, we need to also reduce the rear roll angle by the same amount. So, the teams have found that they need to increase the right-rear spring rate a great deal and create a large spring split in the rear of the car to reduce the rear roll angle.

In past articles, we have explained the dynamics of a straight-axle suspension system. The lateral force vector is in a direction that is down and to the right, usually pointed between the rear tire contact patches. If we have a spring split in the rear, the effect of the spring split will cause either an increase or decrease in the rear roll angle, depending on which side is softer.

If the left-rear spring is softer than the right-rear spring, the car will roll less. If the right-rear spring is softer than the left-rear spring, the car will roll more. So, in order to reduce the rear roll to match the front stiffness, we see rear spring rates that might be as follows: left-rear = 175 and right-rear = 300. Some teams will run the same spring on the left-front, right-front, and right-rear corners and a much softer spring on the left-rear corner.

Note: The spring rates shown will not necessarily work for your particular car. Develop your own setups based on the information from your car. We do not recommend you use these spring rates. They are being used as an example of the general trends in setups.

The problem that has surfaced when using the bigger sway bars in conjunction with softer front springs is the search for balance. Even if a balanced BBSS setup is a bit faster than a balanced conventional setup, the balanced conventional setup will always beat an unbalanced BBSS setup and vice versa. The unbalanced situation is reversed in most cases with the BBSS setups. In the “old” days, our setups were unbalanced such that the front tended to roll less than the rear, putting excess load on the right-front tire. With BBSS utilizing a stiff right-rear spring, if the Panhard bar is not set correctly, the front may tend to roll more than the rear, putting excess load on the right-rear tire. In reality, it can easily go either way, so balance does matter and may be harder to recognize with the BBSS setups.

The RaceTrack Matters The type of racetrack will largely determine the success of the BBSS setups. Usually, the higher-banked half-mile or shorter tracks do not require the BBSS setups. If the track has quick transition of its banking angle from turn to straightaway, a large bar might not allow the car to adjust to the change.

Flatter half-mile tracks and larger-distance flat tracks offer an opportunity to benefit from the more aero-efficient attitude of the car for better downforce and less drag. In any event, remember that the car must be balanced so that each end is working in unison and all four tires are providing maximum grip, and then your car will be as fast as it can be.

Several years ago, the Busch Series championship was won by a team that ran a very stiff right-rear spring at nearly every track. At that point in time, the sway bar rates had not reached the size that they are now. But, nonetheless, that car was more balanced than most of the other cars it was competing against. The setup used might have looked something like this without the large sway bar. Teams who race cars similar to the Busch cars might be running setups similar to this one with the larger sway bar.

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